Once you've decided to push beyond the exceptional and become an Expert Gamemaster, then it's time to take a step back to take a broader view of Gamemastering, and, once you have that view, ask yourself what refinements you can make to your craft.
Out of the entire group, the Gamemaster is the only person who must bring craft to the table, and it is the quality of that craft that separates the Expert Gamemaster from the merely Exceptional. So, to become an Expert Gamemaster, you must look outside of the play of the game, and begin to study the game's design - the design of the rules, the design of your adventures, and the design of your overarching story.
An Expert Gamemaster does more than just learn the rules of the game, they work to understand them. They work to understand how those rules influence one another, and how those rules influence the nature of the game.
This is the third in the series of GM mastery series of How to Book.